Datasheet LT3575 (Analog Devices) - 8

LT3575
APPLICATIONS INFORMATION ERROR AMPLIFIER—PSEUDO DC THEORY
In combination with the previous VFLBK expression yields an expression for V In the Block Diagram, the R OUT, in terms of the internal reference, REF (R4) and RFB (R3) resistors programming resistors, transformer turns ratio and diode can be found. They are external resistors used to program forward voltage drop: the output voltage. The LT3575 operates much the same way as traditional current mode switchers, the major difference ⎛ R ⎞ ⎛ 1 ⎞ FB being a different type of error amplifi er which derives its V = V V I E ( S OUT BG R R) ⎝⎜ R ⎠⎟ α N F SEC ⎝⎜ ⎠⎟ − − feedback information from the fl yback pulse. REF PS Operation is as follows: when the output switch, Q1, Additionally, it includes the effect of nonzero secondary turns off, its collector voltage rises above the VIN rail. The output impedance (ESR). This term can be assumed to amplitude of this fl yback pulse, i.e., the difference between be zero in boundary control mode. More details will be it and VIN, is given as: discussed in the next section. VFLBK = (VOUT + VF + ISEC • ESR) • NPS
Temperature Compensation
VF = D1 forward voltage The fi rst term in the VOUT equation does not have a tem- ISEC = Transformer secondary current perature dependence, but the diode forward drop has a signifi cant negative temperature coeffi cient. To compen- ESR = Total impedance of secondary circuit sate for this, a positive temperature coeffi cient current NPS = Transformer effective primary-to-secondary source is connected to the RREF pin. The current is set by turns ratio a resistor to ground connected to the TC pin. To cancel the The fl yback voltage is then converted to a current by temperature coeffi cient, the following equation is used: the action of RFB and Q2. Nearly all of this current fl ows δV R F FB 1 δVTC through resistor R = − • • or, REF to form a ground-referred voltage. δT R N TC PS δT This voltage is fed into the fl yback error amplifi er. The R − 1 δV R fl yback error amplifi er samples this output voltage R FB TC FB TC = • • ≈ information when the secondary side winding current is NPS δ δ δ VF / T T PS N zero. The error amplifi er uses a bandgap voltage, 1.23V, as the reference voltage. (δVF/δT) = Diode’s forward voltage temperature coeffi cient The relatively high gain in the overall loop will then cause the voltage at the R (δVTC/δT) = 2mV REF resistor to be nearly equal to the bandgap reference voltage VBG. The relationship between VTC = 0.55V VFLBK and VBG may then be expressed as: The resistor value given by this equation should also be ⎛ V ⎞ V verifi ed experimentally, and adjusted if necessary to achieve α FLBK BG or, ⎝⎜ R ⎠⎟ = R optimal regulation overtemperature. FB REF ⎛ The revised output voltage is as follows: R ⎞ ⎛ 1⎞ V = V FB FLBK BG ⎝⎜ R ⎠⎟ ⎝⎜ α⎠⎟ ⎛ R ⎞ ⎛ 1 ⎞ REF V = V FB V OUT BG ⎝⎜ R ⎠⎟ N F ⎝⎜ α⎠⎟ − α REF PS = Ratio of Q1 IC to IE, typically ≈ 0.986 ⎛ V ⎞ R V TC FB BG = Internal bandgap reference − I ESR ⎝⎜ R ⎠⎟ • – ( ) N SEC α TC PS 3575f 8